As a method of bonding different materials such as a ceramic base material (ceramic base) and a metallic member to each other, a method such as one using a solder material can be used. However, during a cooling process after high-temperature bonding, thermal stress caused by a difference in thermal expansion coefficient between the different materials or between the solder material used for bonding the different materials together and the materials occurs. This causes a separation at the interface between the materials or causes cracks in the vicinity of the interface if one of the materials is brittle, so that desired bonding strength and air tightness cannot be obtained in some cases. Since products (different materials bonded member) having the aforementioned troubles broken during a production process must be discarded as defective products, there is an unfavorable increase in production costs. Further, if the product is subjected to thermal cycles in use, for example cycling using a high-temperature heater and so on, these troubles occur after use causing a deterioration of reliability of the product.
As a method of bonding a ceramic base and a metallic member to each other by use of a solder material, there is available a method in which the bonding surface of the ceramic base is bonded to the metallic member after a metallization treatment with a vapor of Mn or Cr, etc. in order to ensure wettability. Another wettability improving method is available in which a reaction layer such as a nitride or an oxide, etc. is formed on the bonding surface of the ceramic base by adding Ti and so on into the soldering material instead of plating.
Meanwhile, in these methods, unless some measures are taken against thermal stress which occurs at the interface between the bonded materials so as to reduce the thermal stress, cracks are often formed in a base material, which is vulnerable to the thermal stress and separation occurs at the interface. That is, not only can bonding strength between the bonded materials be influenced, but also various other properties that are required from composite bonded members under use in specific fields, such as air tightness, may be influenced. Particularly, it is very difficult to bond a low-strength base material such as aluminum nitride and a member composed of metal or the like to each other while reducing the occurrence of the above problems.
To solve the above problems, there is available a method of bonding a ceramic base and a metallic member together through liquid-phase bonding using a solder material as a bonding material comprising a metal of low proof stress such as Au which undergoes plastic deformation by a low stress. However, in a case where a metallic member comprises Ni, Co or metal comprising any one thereof, for example Kovar, the metal itself, that is Ni, Co or its constituent, e.g. Fe, diffuses into Au constituting the solder material, thereby increasing the proof stress of Au. As a result, a thermal cycle or a thermal shock may crack the base material.
Moreover, when, for example, Kovar is used as the metallic member, in the case of bonding Kovar to the ceramic base, the components constituting the Kovar (Fe, Ni, Co) diffuse into the solder material to form an intermetallic compound layer of low electrical conductivity, which causes deterioration of thermal cycle characteristics and abnormal heat generation at that part.
On the other hand, use of a metal which does not form a solid solution with Au, such as W, Mo or the like, as the metallic member can also be considered. However, these metallic materials are severely oxidized under high temperature conditions in the air, and cannot be used as metallic members for high-temperature heaters which are exposed to such conditions.
For solving the above problems, it has been attempted to devise the bonding structure. For example, JP-A-10-209255 discloses a bonding structure of a ceramic base 1 and a connector for a power supply 16 as shown in FIG. 4 as a susceptor for disposing a semiconductor wafer. A hole 14 is provided in a ceramic base material 1. From the hole 14, a metallic member 17 comprising a metal such as Mo which has a thermal expansion coefficient approximate to that of the ceramic base material 1, is embedded with a portion thereof beforehand. Further, in the hole 14, a cylindrical atmosphere protector 10 is inserted, and inside the protector 10, a connector 16 for supplying power and a low thermal expansion material 15 are inserted. The protector 10 and the connector 16 are hermetically bonded together with a solder material 5, and the material 15 and the protector 10 are hermetically bonded to the metallic member 17 with the solder material 5.
According to this bonding structure shown in FIG. 4, residual stresses at the time of bonding material 15 and the metallic member 17 together are relaxed, and oxidation of the metallic member 17 is also restrained by the atmosphere protector 9. Hence, even if bonding is carried out by use of a solder material of high proof stress such as the Au-18Ni solder material, no cracks occur in the ceramic base material 1 at bonding. Moreover, since the strength change caused by heat is little as well, the endurance reliability when the bonded interfaces are exposed to a thermal cycle or a thermal shock. However, the foregoing bonding structure has such problems that the number of parts is large and that a very high production control ability is required, because deterioration of the metallic member 17 occurs due to its oxidation unless the atmosphere protector 10 and the metallic member 17 are fully bonded to each other.
Further, JP-A-11-278951 discloses the structures of a corrosion-resistant metal ring 23 and a susceptor 22 such as one shown in FIG. 6 or 7 so as to relax thermal stress which occurs when a corrosion-resistant metal ring 23 made of Kovar or the like is bonded to the back side 22b of a ceramic susceptor 22 for disposing a semiconductor wafer 24 installed in the chamber 21 of a semiconductor container with the structure shown in FIG. 5. Namely, employing such structure of members (corrosion-resistant metal ring 23 and susceptor 22) is effective for relaxation of thermal stress. However, bonding a ceramic base 1 and a corrosion-resistant metal ring 23 with a melted solder material 26 causes the metal component constituting the corrosion-resistant metal ring 23 to be eluted into the solder material 26 and the solder material 26 tends to be deteriorated. Nothing but the consideration about the shape of the members shown in FIGS. 6 and 7 bring insufficient effect for relaxation of thermal stress, and a defect such as a breakage of ceramic base 1 may occur.